Collision protecting system for TLP structures

ABSTRACT

A light weight, energy dissipation bumper system for protecting the columns of an offshore platform. A plurality of rows of crushable tubes are provided to dissipate collision energy through absorption without storing such energy. The tubes have a diameter to thickness ratio between 30 and 40 for mild steel providing neutral/positive buoyancy. The tubes may be easily replaced when their ability to protect the column to which they are attached to exhausted through flattening. The tubes may extend either axially along or radially outward from the column.

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to fenders for a relatively stationaryoffshore platform More particularly, the present invention is directedto a light weight, energy-dissipation bumper system for the columns ofan offshore tension leg platform, or the like.

Many sophisticated (and, therefore, expensive) pile protector systemshave been devised. Some of these have possible application on offshoreplatforms such as tension leg platforms, and the like. Among thesebumper systems are those shown in U.S. Pat. Nos. 3,503,600 to Rich;3,541,800 to Walker et al.; 4,032,126 to Laughlin et al.; 4,398,848 toGuilbeau; 4,494,738 to Britton et al.; and 4,650,371 to Sawaragi et al.

Besides being overdesigned and, therefore, not cost effective solutions,a number of these systems would impose undue weight penalties on thefloating platform. While not all possible applications for these bumpersare weight sensitive, offshore platforms are particularly susceptible toconsiderations of weight. Indeed, ss a general rule, for each pound ofweight positioned above the waterline, two pounds of structure arerequired below the waterline to support it. Hence, any bumper systemutilized to protect the columns of an offshore platform that does nothave weight considerations among its most significant design criteria,will not have satisfactory performance.

The energy-dissipation bumper system of the present invention has twoprincipal design features: (1) light weight, and (2) simplicity, both ofwhich precipitate low cost. First and second coaxial cylindrical shellssandwich a plurality of light-weight crushable tubes. These tubes absorbthe energy of a potentially destructive collision solely by beingcrushed. Tubes which become so damaged as to inadequately protect thecolumn structure to which they are attached can be easily replaced Thesetubes may either extend radially about the column or axially along thecolumn in a plurality of rows. By preventing ingress of seawater intothe bumper system, the hollow crushable tubular members actually providebuoyancy to the platform, as opposed to imposing a weight penalty.

Various other features advantages and characteristics of the presentinvention will become apparent after a reading of the following detaileddescription.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagrammatic side view of a tension leg platform utilizingthe column protecting bumper system of the present invention;

FIG. 2 is a cross-sectional top view of a column equipped with oneembodiment of the bumper system of the present invention;

FIG. 3 is an enlarged perspective view of a second embodiment of thepresent invention with parts broken away.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A tension leg platform of the type with which the energy-dissipationbumper system of the present invention might be used is shown in FIG. 1generally at 10. This floating drilling and production facility ismoored to the sea floor 14 by means of tendons 12 which engage inreceptacles formed on foundation template 13 Template 13 is secured tothe ocean floor 14 by piles 15 which are driven, or otherwise secured,therein. Upper (16) and lower (17) decks are secured to a plurality ofcolumns which are held together by pontoons 19. An axial portion of eachcolumn 18 adjacent a minimum water line 23 is protected from collisionby ships, delivery boats, debris, etc., by an energy-dissipation bumpersystem 20 secured about a portion of the periphery thereof. It ispreferred that the bumper system 20 protect a region from 20 feet belowto 25 feet above water line 23. Production tubing inside of risers 21connect subsea wells with wellheads on deck 17.

A first embodiment of the energy-dissipation bumper system 20 of thepresent invention is shown in greater detail in FIG. 2. Bumper system 20is comprised of a first inner shell 22 with a first outer diameter and asecond coaxial outer shell 24 with a second larger diameter. Sandwichedbetween shells 22 and 24 is a plurality of crushable thin-walled tubes26 which extend substantially parallel to the axes of shells 22 and 24.As depicted in FIG. 2, each of the tubes 26 has the same outer diameterand wall thickness and the tubes 26 are deployed in a five spot pattern(i.e., adjacent rows are circumferentially offset). This configurationwill permit some of the energy to be dissipated as friction resultingfrom positional readjustment of tubes 26 as a result of a collision withouter shell 24. However, the substantial majority of the collisionenergy will be dissipated (i.e., absorbed) by the collapse of the tubesas they are crushed under the load. While it would be possible for innershell 22 to be the outer skin of column 18, it is preferred that aseparate layer be used to form shell 22 to ensure the integrity ofbumper 20 and ease of attachment.

The trailing edges 28 of bumper system 20 are tapered back into column18 to minimize drag loading by wind and waves. A top (30) and bottom(32) panel completely enclose tubes 26 in a sealed compartment.Accordingly, the hollow tubes 26 can provide buoyancy to the bumpersystem 20. In fact, by properly selecting the proper wall thickness anddiameter as well as length of tubing 26 that is submerged, for aselected material the bumper system 20 can be neutrally buoyant or evenslightly positively buoyant. Thus, instead of imposing a weight penalty,the bumper system 20 of the present invention can fully support its ownweight. For mild steel, the preferred material for constructing tubes26, the diameter to wall thickness ratio must exceed 29.5 forneutral/positive buoyancy and preferably is in the range of from about30 to about 40.

In the FIG. 2 embodiment, by way of example and not limitation, tubes 26may have an outer diameter of 12 inches and a wall thickness of 3/8inch. Preferably, tubes 26 would be constructed in five foot lengthswith telescopically interconnecting ends to facilitate replacement ofdamaged sections without the need to scrap an entire tube length.Similarly, inner and outer shells 22 and 24, respectively, can beconstructed to permit easy access as needed, for example, forrepair/replacement of crushed tubes following a sufficient number ofcollisions to render the fender 20 incapable of adequately protectingcolumn 18. Tack welds may be used between adjacent tubes 26 and betweentubes 26 and shells 22 and 24 to maintain the tubes in position tofurther facilitate replacement. Lastly, outer shell 24 can havesufficient malleability to permit it to be easily popped back into itsoriginal shape as repair is being effected after a collision.

A second embodiment of the bumper system 20 of the present invention isdepicted in FIG. 3. In this embodiment, the crushable tubes 26 extendgenerally radially outwardly between inner shell 22 and outer shell 24.Intermediate row 27 between rows 25 and 29 may be offset as in the FIG.1 embodiment and further, may be of a second smaller diameter to fillthe gaps between the tubes 26 of rows 25 and 29. As the tubes extendgenerally radially outwardly, they will form spaces between their outerperimeters. By way of example, a 6" diameter tube extending outwardlytwo feet will have 5/8" between it and any of the four closest adjacenttubes. The interspersion of the smaller diameter tubes willsignificantly rigidify the structure of bumper 20. It is possible thatthe axial tubes of the FIG. 3 embodiment may produce too stable astructure to perform as a bumper, depending on the material selected toform the crushable tubes. In this case, the outer surfaces of tubes 26could be scored to permit a set of preferred tearing lines to facilitatethe axial crushing. This may be necessary to prevent the tubes frompenetrating (spear-like) inner (22) and outer (24) shells uponcollision.

In the FIG. 3 embodiment, again, by the way of example, the firstplurality of tubes could have a diameter of 12 inches and a wallthickness of 3/8 inch and the second plurality an outside diameter of 5inches and a wall thickness of 1/8 inch. When two diameters of tubes areused the ratio of the large diameter to small diameter tubes could befrom about 1.5 to 6 and in no event should the ratio exceed 10. It willbe appreciated that the dimensions specifically called out here are forpurposes of example only, since the selection of actual dimensionscannot be done in an isolated manner outside the context of total systemdesign as the particular design parameters of a given situation require.

Various changes, alternatives and modifications of these embodimentswill become apparent to the person of ordinary skill in the artfollowing a reading of the foregoing specification. Accordingly, it isintended that all such changes, alternatives and modifications as fallwithin the scope of the appended claims be considered part of thepresent invention.

I claim:
 1. An energy-dissipation bumper system for protecting a portionof an outer periphery of a column of a tension leg platform, said bumpersystem comprisinga first inner cylindrical shell; a second outercylindrical shell surrounding said first cylindrical shell and beinggenerally coaxial therewith; a plurality of non-resilient, crushable,thin-walled tubular members lying between said first and said secondcylindrical shells, said tubular members being constructed of tubularsteel and having a diameter to wall thickness ratio of between about 30and about 40, said tubular members having a sufficiently large radialextent to permit substantially all of the energy from an impact withsaid bumper system to be dissipated by absorption due primarily tocollapse of said crushable thin-walled tubular members without damage tosaid first cylindrical shell.
 2. The energy-dissipation bumper system ofclaim 1 wherein said crushable thin-walled tubular members extendgenerally radially outwardly from said first inner cylindrical shell. 3.The energy-dissipation bumper system of claim 1 wherein said crushablethin-walled tubular members extend generally axially about the peripheryof said first cylindrical shell
 4. The energy-dissipation bumper systemof claim 3 wherein said axially-extending thin-walled crushable tubularmembers comprise a plurality of layers of said members surrounding atleast a portion of said first cylindrical shell.
 5. Theenergy-dissipation bumper system of claim 4 wherein one of saidplurality of layers is offset circumferentially from at least one otherof said layers.
 6. The energy-dissipation bumper system of claim 1wherein said crushable thin-walled tubular members encircle only aportion of a circumference of said tension leg platform column.
 7. Theenergy-dissipation bumper system of claim 6 wherein said portion of saidcircumference of said column comprises generally from about five-eighthsto about two-thirds of a circumferential periphery of said column. 8.The energy-dissipation bumper system of claim 7 wherein said portion ofsaid column protected by said crushable thin-walled tubular members islimited in axial extent to a region immediately adjacent a normal waterline.
 9. The energy-dissipation bumper system of claim 8 wherein saidaxial region comprises a zone extending from about 20 feet below theminimum water line to a point 25 feet above said normal water line. 10.The energy-dissipation bumper system of claim 1 wherein each of saidplurality of crushable, thin-walled tubular members has generally anoutside diameter which is generally about the same.
 11. Theenergy-dissipation bumper system of claim 1 wherein said plurality ofcrushable, thin-walled tubular members include a first group ofcrushable members having a first diameter and a second group ofcrushable members interspersed with said first group having a secondsmaller diameter.
 12. The energy-dissipation bumper system of claim 11wherein the ratio of said first diameter to said second diameter lies inthe range of from about 1.5 to about
 6. 13. An energy-dissipation bumpersystem for protecting a portion of an outer periphery of a column of atension leg platform, said bumper system comprising a first innercylindrical shell; a second inner cylindrical shell surrounding saidfirst cylindrical shell and being generally coaxial therewith; aplurality of non-resilient, crushable, thin-walled tubular members lyingbetween said first and said second cylindrical shells including a firstgroup having a first diameter and a second group interspersed with saidfirst group having a second smaller diameter, the ratio of said firstdiameter to said second diameter lying in the range from about 1.5 toabout 6.0, said tubular members having a sufficiently large radialextent to permit substantially all of the energy from an impact withsaid bumper system to be dissipated by absorption due primarily tocollapse of the crushable thin-walled tubular members without damage tosaid first cylindrical shell.